Why does cube UV unwrap at an angle? The mesh is square but the UV is skew, at an angle
I've checked out your seams placement and you're right- it doesn't unwrap in the right way (don't know why). To solve it you may snap the cursor (2D Cursor mode) to the very bottom vertex with Shift+S-->Cursor to Selected, select the nearest one and rotate them to create a straight line (you may then scale the vertices by Y axis- S,Y,0 to make sure the edge in a perfect horizontal position). Then pin the vertices with P. Finally unwrap the mesh again (U-->Unwrap) and get rid of pinning (Alt+P).


You may also try to place  the seams in a different way e.g. as pictured below. As you can see it unwraps well now.